Connected: An Internet Encyclopedia
RFC 922 - 7. Broadcast IP Addressing - Conventions

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### 7. Broadcast IP Addressing - Conventions

If different IP implementations are to be compatible, there must be convention distinguished number to denote "all hosts" and "all subnets".

Since the local network layer can always map an IP address into data link layer address, the choice of an IP "broadcast host number" is somewhat arbitrary. For simplicity, it should be one not likely to be assigned to a real host. The number whose bits are all ones has this property; this assignment was first proposed in [6]. In the few cases where a host has been assigned an address with a host-number part of all ones, it does not seem onerous to require renumbering.

The "all subnets" number is also all ones; this means that a host wishing to broadcast to all hosts on a remote IP network need not know how the destination address is divided up into subnet and host fields, or if it is even divided at all. For example, 36.255.255.255 may denote all the hosts on a single hardware network, or all the hosts on a subnetted IP network with 1 byte of subnet field and 2 bytes of host field, or any other possible division.

The address 255.255.255.255 denotes a broadcast on a local hardware network that must not be forwarded. This address may be used, for example, by hosts that do not know their network number and are asking some server for it.

Thus, a host on net 36, for example, may:

- broadcast to all of its immediate neighbors by using 255.255.255.255

- broadcast to all of net 36 by using 36.255.255.255

without knowing if the net is subnetted; if it is not, then both addresses have the same effect. A robust application might try the former address, and if no response is received, then try the latter. See [1] for a discussion of such "expanding ring search" techniques.

If the use of "all ones" in a field of an IP address means "broadcast", using "all zeros" could be viewed as meaning "unspecified". There is probably no reason for such addresses to appear anywhere but as the source address of an ICMP Information Request datagram. However, as a notational convention, we refer to networks (as opposed to hosts) by using addresses with zero fields. For example, 36.0.0.0 means "network number 36" while 36.255.255.255 means "all hosts on network number 36".

Next: 7.1. ARP Servers and Broadcasts

Connected: An Internet Encyclopedia
RFC 922 - 7. Broadcast IP Addressing - Conventions